The present invention is directed to thin films or coatings, particularly to thin boron containing hard coatings, and more particularly to multi-phase boron nitride, cubic boron nitride, multilayer boron/boron nitride, and multilayer boron/cubic boron nitride, and to the fabrication thereof.
Thin foils are widely used, for example, as band-pass filters, in transmission filters, and in spectroscopic applications which use irradiation wavelengths in the range of extreme ultra-violet to soft x-rays. Submicron foils, 0.1-0.2 .mu.m thick, have been produced in various low Z materials (low atomic number) (i.e. carbon, beryllium) by sputtering and evaporation processes. Boron films have been previously produced by resistance heating or electron beam evaporation (Labov, S. et al. Applied Optics 24: 576 (1985)). Previous efforts to prepare boron foils, films, or coatings by sputter deposition were precluded by the lack of availability of dense, high purity sputter targets. However, boron films have now been produced by a magnetron sputtering method described in above-referenced U.S. Pat. No. 5,203,977. Typically sputtered boron films exhibit superior mechanical properties and are preferred because they have fewer defects and finer morphological growth features than foils prepared by evaporative processes.
Boron nitride (BN) films have been produced by a variety of ion-assisted vapor deposition processes including: chemical vapor depositions, evaporation, rf sputtering and ion beam deposition. Boron nitride films prepared by these ion-assisted deposition processes (i-BN) typically are nano crystalline with significant amounts of the more stable hexagonal phase (h-BN). The deposition rate of BN films decreases with increasing ion bombardment.
The preparation of BN films containing cubic boron nitride (c-BN) requires plasma or ion bombarding enhancement of these processes. Also, the previously produced BN films containing appreciable amounts of the cubic phase are highly stressed and reported unstable in air.
The crystalline phases of boron nitride (BN) are analogous to those of carbon. Cubic boron nitride (c-BN), defined as BN with 80% cubic phase, is similar in structure to diamond, and it shares many of diamonds properties including: high thermal conductivity, hardness, high dielectric constant and good chemical stability. Cubic boron nitride is unusual in that it has a wider band gap than any III-V or IV semiconductor, and it can be P or N doped. Thin films of c-BN are potentially useful as hard coatings for tribological applications and cutting tools, as a protective insulator on semiconductors, for optical surfaces in severe environments and for semiconductor devices operating at high temperatures.
Multilayered coatings, which are used, for example, as reflective layers in x-ray optics, are typically tens to hundreds of angstroms thick. A multilayer x-ray mirror is the analog of a quarter-wave stack reflective coating with the added complication of radiation absorption in the layers. Physically, it is an alternating sequence of thin films of highly absorbing and less absorbing materials deposited on an optically smooth substrate. The layered structure is periodic and results in a large angle, resonant reflectivity which is three or four orders of magnitude greater than the simple Fresnel reflection from an unlayered surface. Reflectivity in a multilayer mirror derives from the interference of x-rays coherently scattered from the interfaces between materials of higher or lower x-ray absorption.
The quality of the multilayered optical coating is determined by the perfection of the interfaces between the layers and the uniformity of the layer dimensions. Standard methods for application of multilayer coatings use the physical vapor deposition (PVD) process of evaporation or sputtering.
The coarse layer microstructure produced and the inherent difficulty in controlling the evaporation processes adversely effects the interface perfection and layer dimensional stability, and consequently, the efficiency of the optical coating produced by such methods. The use of computer controlled sputtering processes, such as rf magnetron sputtering process, allows the production of complex multilayer coatings with variable layer thickness and composition. A method for forming multilayer structures using an rf magnetron sputtering process is described in above-referenced U.S. Pat. No. 5,203,977, and titanium/boron (Ti/B) multilayers are described and claimed in above-referenced, copending U.S. application Ser. No. 08/048,373, filed Apr. 15, 1993, now U.S. Pat. No. 5,389,445, entitled "Magnetron Sputtered Boron Films And Ti/B Multilayer Structures".
With the advent of the magnetron sputtering deposition process and the development of high density, crystalline boron targets, it has been found that amorphous boron films which have no morphological growth features can be produced. Such are described and claimed in aboved referenced copending U.S. application Ser. No. 08/334,090 filed Nov. 4, 1994, entitled "Magnetron Sputtered Boron Films". The availability of boron sputter targets has provided the unique opportunity to evaluate the preparation of multi-phase boron nitride, particularly c-BN films by reactive sputtering boron in a partial pressure of N.sub.2 or a nitrogen containing gas, in accordance with the present invention. The term "boron nitride" as set forth hereinafter defines a material which is composed of multi-phase boron nitride which includes the i-BN, h-BN, and c-BN phases, while the term cubic boron nitride (c-BN) defines a material which has at least 80% of the cubic phase. It has been found that boron nitride and cubic boron nitride films or coatings can be produced by a reactive sputtering process similar to that used in producing the boron films by utilizing the high purity boron target and a nitrogen containing environment or atmosphere during deposition. This constitutes a new type of BN forming process capable of producing stress-free, single phase films of c-BN. Also, by combining these magnetron sputtering processes, multilayers of boron/boron nitride and boron/cubic boron nitride can be produced. This is accomplished by simply alternating the gas composition from pure argon to a nitrogen containing gas while sputtering from a single boron target. The multilayer films and coatings of boron and boron nitride or cubic boron nitride are a new type of composite hard coatings. It provides a unique method of controlling the stress when the boron nitride layer is predominantly cubic phase (c-BN). These thus produced films or coatings, may be utilized effectively in various electronic components, as well as providing hard surface coatings for tools, machinery equipment, and engine components, etc. Thus, by the discovery of a process for producing boron, boron nitride, cubic boron nitride, and/or multilayers of these materials, prior problems have been addressed, especially stress, and needs particularly for hard coatings, have been provided by this invention, thus greatly advancing the state of the art.